1. Field of the Invention
The invention relates to a belt pressing unit and to a method of guiding a stream of cooling and lubricating liquid through the inside of a belt pressing unit.
2. Description of the Prior Art
A belt pressing unit is a pressing device which is preferably used as the wet press of a paper machine and which has a so-called extended press nip or zone. This means that the press nip is formed between a normal press roll, or back roll, on the one hand, and an elastic tubular press belt on the other hand. The press belt is adapted to be pressed against the back roll by means of a pressing device, such as a press shoe. The resulting press nip is relatively long in the circumferential direction so that pressure is exerted on the web of paper over a greater length than in a conventional roll press consisting of two rolls. The web of paper passes through the press nip together with a felt dewatering belt.
The belt press unit disclosed in German Patent Application DE-OS No. 31 26 492 has a box-shaped supporting body. Except in the press zone around the press nip, the tubular press belt travels around the body at a large distance away from the body. Lubricating liquid is fed directly onto the support surface of the pressing device. The lubricating liquid which escapes from the press zone is collected in the lower region of the inner space defined by the press belt and is removed via a discharge conduit extending through the supporting body to the outside. One disadvantage of this arrangement is that the lubricating liquid fed into the press zone is greatly heated there and thus cannot cool the press belt significantly. There is a danger that the press belt will reach an impermissibly high temperature after an extended period of operation.
The belt press units disclosed in German Patent Application DE-OS No. 31 02 526 have a substantially roll-shaped supporting body. In the arrangement shown in FIG. 1, the press belt revolves through most of its path at a slight distance from the outer surface of the supporting body. In the arrangement shown in FIGS. 4 and 5, the press belt slides over the supporting body. In the latter arrangement, there is a danger that a large amount of additional frictional heat will be produced. A large number of lubricating chambers are distributed along the circumference of the outer cylindrical surface of the supporting body for feeding a lubricating liquid. This ensures that lubricating liquid is fed to the press belt at a number of successive points in each revolution. Nevertheless, due to the large amount of frictional heat produced, substantial heating of the press belt must be expected. Outside the press zone, a sliding shoe is provided in the supporting body, which shoe cooperates with a drive roll and can also be used to tighten the press belt.
In the arrangement disclosed in U.S. Pat. No. 4,287,021, the press belt passes through most of its path around the supporting body at a slight distance from the body. A device for feeding lubricating liquid is provided just before the point where the press belt enters the press zone. In this way, the travel surface of the press shoe which transmits the pressing force to the press belt is the only surface provided with lubricating liquid. This liquid is to a large extent removed from the inner surface of the press belt by a scraper after the press belt leaves the press zone. In addition, the supporting body has openings a slight distance after the press zone so that any lubricating liquid still adhering to the press belt at that point can flow back into the inside of the supporting body.
German Patent Applications DE-OS No. 31 26 492, discussed above, and DE-OS No. 19 23 784 and U.S. Pat. No. 3,269,893 disclose that the tubular press belt, together with two tensioning disks at its ends, can form an inflatable tube roll with a closed inner space which can be inflated by compressed air. Such a closed inner space is also provided in the arrangement disclosed in German Patent Application DE-OS No. 31 02 526, discussed above, but not in the arrangement disclosed in U.S. Pat. No. 4,287,021, discussed above, since in the latter arrangement the cross-section of the supporting body differs too much from a circular shape. In that arrangement, the inner space surrounded by the press belt is open to the outside at both ends. Since the danger thus arises that lubricating oil will escape and come in contact with the web of paper, for instance, the inside of the press belt has diagonally extending grooves for conveying the lubricating liquid from the ends toward the inside.
The arrangement in which the press belt defines a closed inner space has been known now for more than twelve years. A substantially higher solid content in a web of paper is obtained by draining with a wet press with an extended press nip at the press end than by draining with traditional roll presses. Therefore, a considerable amount of energy can be saved during the subsequent thermal drying of the web of paper. Nevertheless, a belt press unit as disclosed in German Patent Applications DE-OS No. 31 26 492, DE-OS No. 31 02 526 and DE-OS No. 19 23 784 and U.S. Pat. Nos. 4,287,021 and 3,269,893, discussed above, has not been actually employed in the press end of an industrial paper machine, to the best of applicant's knowledge. Only the type of arrangement disclosed in U.S. Pat. No. 4,272,317 is actually used in wet presses with extended press nips. In that arrangement, the press belt travels over several guide rolls, some of which are mounted on swing levers for tensioning the press belt. The structural expense of that arrangement is greater than that of a tubular press belt. Furthermore, the danger arises again that lubricating liquid will escape.
Numerous design requirements have prevented the use of a belt-pressing unit with tubular press belt in wet presses up to now, including the following:
1. The press belt must not twist or form wrinkles during its revolution. In other words, the points of an imaginary line extending transverse to the direction of rotation on the press belt must all move with exactly the same speed.
2. The amount of drive energy required for the rotation of the press belt should be as small as possible. This applies also to the start-up of the belt press unit; i.e., the starting torque should be as small as possible. As a rule, a separate drive is not provided for the press belt. Rather, it is carried along by the felt belt.
3. The life of the press belt and of the press shoe should be as long as possible. For this, the heat produced must be effectively removed, among other things. A tubular press belt can give off only a very small amount of heat to the surrounding air since its circumference is substantially less than the circumference of a press belt which travels over rolls, as in U.S. Pat. No. 4,272,317, discussed above.
4. The above-mentioned requirements must be met for the customary dimensions in modern paper machines, including a work width of up to 10 m and travel speeds of the order of 1000 m/min.